This invention relates to ground fault detectors and shutdown systems, and particularly to a ground fault detection and shutdown system for underwater power transmission to prevent divers from exposure to electrical shock hazards.
On land, the operator or user of an electrical power system must come into direct contact with the electrical system and become part of the circuit to be injured. However, underwater a diver does not have to make contact with the electrical circuit to be injured, since a break in the system insulation can allow stray electrical currents to flow through the water. Such stray currents can shock a diver if he swims into the leakage path.
As tools for underwater use by divers become more sophisticated and larger work systems developed, more power to operate these devices is required. Due to the flow losses and handling problems associated with large hydraulic power systems, other power sources must be developed. Electrical power can be used underwater to fill this need if used with properly designed protection systems. However, stringent safety systems are required to protect divers from electric shock.
There are two ways a diver can receive an electrical shock: by direct contact with a live conductor, and by swimming into an electrical field created by a leakage current in the water. Either of these circumstances can cause the diver to suffer an involuntary physical reaction. The seriousness of the effects of electrical shock depends upon the frequency of the current, the magnitude of the current, the current path through the body, and the shock duration.
The physical effects of electrical shock represent only a part of the problem facing the diver. For example, an involuntary physical reaction to very small leakage currents could cause him to drop a tool or release a heavy object, injuring himself or others. Larger shock currents that result in respiratory arrest obviously present a very severe hazard to the diver. Since the diver must be taken to the surface for treatment, many factors must be considered prior to surfacing, such as depth of the dive, time at depth, and decompression schedule. For these reasons, any electrical systems used by divers must incorporate an effective ground fault detector system. The ground fault detector system should eliminate even the diver's perception of being shocked in case of a ground fault in the electrical power system.
All of the methods for protecting divers from electrical shock fall into one of two categories: passive and active. Passive methods of protection are: insulation; double insulation; screening (shielding); earthing (grounding); and isolation. Active methods of protection are: earth-leakage circuit breakers; earth continuity monitor; line insulation monitor; detection of primary defect where two defects are needed to present a shock hazard; fail-safe design; and, quality control and preventive maintenance to ensure reliability.
Reliable protection from shock hazards cannot be obtained for divers by passive methods as in dry land systems. Without active monitoring systems to detect water intrusion, passive methods can be defeated and the diver not be aware of his loss of protection.
Prior ground fault detection and/or interruption systems use AC power and monitor AC leakage currents to ground or continuity of the ground circuit relative to the neutral wire. Several prior systems use differential transformers as an AC leakage current sensing element; such devices cannot allow for leakage due to the distributed capacitance of long transmission lines (greater than 100 feet, for example). Other systems monitor the status of the ground path but provide no protection for humans contacting a hot line, or protect only the transmission line and not the operator of equipment it supplies power to.
There are no prior art fault protection systems which meet diver electrical safety criteria. Commercially available ground fault interruptors will not remove power fast enough and are subject to nuisance power shutdowns. Such prior type systems are primarily for terrestrial application and not applicable for underwater power transmission.